Dysprosium (Dy) is conventionally added to a material alloy for the purposes of improving heat resistance of a rare-earth-iron-boron based (R-T-B) sintered magnet, and of maintaining the coercive force high even in a high temperature condition. The Dy is a kind of rare earth element exhibiting an effect of enhancing an anisotropic magnetic field of R2T14B phase as a main phase of the R-T-B sintered magnet. The Dy is a rare element. For this reason, if the practical use of electric vehicles is advanced, and the demand for magnets with high heat resistance used in motors for the electric vehicles is increased, an increase in material cost is a matter of concern as a result of tightening of the Dy source. Therefore, the development of technology for reducing the use of Dy in magnets with high coercive force is strongly required.
Conventionally, Dy is added in such a manner that the Dy is blended and melted together with the other elements in material casting. According to such a conventional method, Dy is uniformly distributed in a main phase of a magnet. However, the mechanism for generating the coercive force of the R-T-B sintered magnet is nucleation type, so that, in order to increase the coercive force, it is important to suppress the generation of opposing magnetic domain in the vicinity of the surface of R2Fe14B crystal grains as a main phase. For this reason, as shown in FIG. 1, if the Dy concentration can be increased in the vicinity of the surface of the main phase (Nd2Fe14B) crystal grains, that is, only in a grain surface region of the main phase, a high coercive force can be realized with a reduced amount of Dy. In FIG. 1, the grain surface region of the main phase in which the Dy concentration is relatively increased is represented as “(Nd, Dy)2Fe14B”. In a grain boundary phase, a rare earth rich (R-rich) phase exists.
As methods of reducing the use amount of Dy, thereby obtaining a structure shown in FIG. 1, a method of adding an oxide of Dy (see J. Magn. Soc. Jpn. 11(1987)235), a method of adding a hydride of Dy (see J. Alloys Compd. 287(1999)206), and the like have been proposed, for example.
However, the above-mentioned method of adding the oxide involves a problem that the magnetization is disadvantageously deteriorated as a result of the increase in the amount of oxygen as an impurity. The method of adding the hydride involves a problem that the degree of sintering is deteriorated.
In order to avoid such problems, many suggestions such as the followings are made for structure control by means of multi-alloy method in which a main phase alloy having a composition closer to the stoichiometric composition of Nd2Fe14B and a liquid-phase alloy of Dy-rich are blended.    (1) Method in which a Dy—Cu alloy is used (Japanese Laid-Open Patent Publication No.6-96928)    (2) Method in which a Dy—Co alloy having a low melting point is used (IEEE Trans. Mag. 31(1995)3623)    (3) Method in which a Dy—Al alloy is used (Japanese Laid-Open Patent Publication No. 62-206802)    (4) Method in which an R-rich R-T-B alloy including B (boron) is used (Japanese Laid-Open Patent Publication No.5-21218)
However, all of the compositions of the Dy alloys used in the above-identified prior arts are rare-earth rich, so that they are easily oxidized during the pulverization or the like. As a result, the amount of oxygen included in the final magnet is increased, so that there exists a problem that the magnetic properties are deteriorated. In addition, since the embrittlement by means of hydrogen occlusion process cannot be efficiently performed for any of the alloys, the degree arid the efficiency of pulverization are bad, and it is difficult to finally obtain fine particles. In addition, in the case where the Dy—Cu alloy or the Dy—Co alloy is used, there exists a problem that the degree of sintering is significantly deteriorated.
A main object of the present invention is to provide a method of suppressing the oxidation of non main-phase alloy, and of improving the ease of pulverization, in a method of producing a permanent magnet obtained by blending a powder of main phase alloy with a powder of non main-phase alloy including a rare-earth element such as Dy which contributes to the improvement of coercive force.